This invention relates to metallurgical furnaces. In one aspect, this invention relates to a forebay attached to a metallurgical furnace while in another aspect, this invention relates to a method of removing a melt from a metallurgical furnace through a forebay. In yet another aspect, this invention relates to using a forebay to physically separate molten slag from molten metal product.
Metallurgical melts (or simply melts) are the molten contents of a metallurgical furnace. The melts of a metallurgical furnace are typically removed or recovered through tapholes in the furnace walls. Melts, particularly the melts of quiescent bath furnaces, typically comprise two relatively immiscible layers, a slag layer and a metal product layer. The metal product layer contains the desired product, e.g. molten copper or nickel matte, molten blister copper, etc., and the slag layer contains the melt waste, e.g. gangue mineral, flux, iron oxides and the like (although in many situations, the slag layer contains a sufficient concentration of metal values that it is recycled or further processed before ultimate disposal). The slag typically has a low specific gravity relative to, and thus floats on top of, the molten metal product.
Various methods exist for separating a slag phase from a metal product phase. Lead and iron furnaces drain both metal product and slag phases through one tap hole into a vessel or containment which is separate and apart from the furnace and in which one phase is separated from the other. In lead metallurgy, this separation vessel is known as a forehearth. In iron metallurgy, this separation containment is known as a runner. In certain copper metallurgical processes, e.g. the Mitsubishi process, the melt of the smelting furnace is transferred by launder to a separation furnace in which the slag phase is separated from the molten matte (e.g. U.S. Pat. No. 5,380,353).
The usual method for removing a melt, at least in most copper and nickel metallurgical processes, is to remove the individual phases separately, and this is usually accomplished by draining the individual phases through tapholes located at different elevations on one or more of the furnace side walls. Slag is "skimmed" from the top of the molten metal product by draining it through tapholes located above the top surface of the molten metal product or, in other words, above the interface between the slag and the molten metal product. The molten metal product is drained from the furnace through tapholes located below the bottom surface of the slag layer or, in other words, below the interface of the slag and molten metal product. This system of using tapholes for removing slag and molten metal product from a metallurgical furnace is not without problems.
One problem is maintenance. The refractory about every taphole, particularly a slag taphole, is subject to accelerated wear relative to the wear experienced by the refractory in other areas of the furnace. Moreover, usually about every taphole and behind the refractory are water-cooled blocks (typically made of cast copper). The purpose of these blocks is to provide protection to and prolong the life of the furnace refractory located about the tapholes. Maintenance, e.g. repair or replacement, of both the internal refractory and the cooling blocks immediately behind the internal refractory can occur only when the furnace is out of operation (as opposed to the external refractory and those blocks behind it which can be serviced while the furnace is in operation) and since these internal refractory and cooling blocks require more frequent attention then do the others of the furnace, this means that the furnace must be taken out of operation more frequently than would otherwise be required if the furnace did not use functioning tapholes.
Another problem associated with tapholes relates to their sealing. When not in use, tapholes must be closed and this is accomplished typically with the injection of clay. When use of the taphole is required, the clay must be removed, typically with an oxylance or drilling apparatus. Both techniques are labor intensive and awkward.
Yet another problem associated with tapholes is the possibility of a "run away" discharge of the metallurgical melt, e.g. slag, molten matte, blister copper, etc. If the tap hole becomes enlarged or misshapened (either or both of which can result from poor execution of the tap hole opening procedure), then stopping the flow of melt from the furnace can be difficult and a run-away can result in which the receiving vessel or the laundering channels overflow. This, in turn, can cause significant damage to the metallurgical equipment and facility, and the clean-up of such a spill can be expensive.
Still another problem associated with tapholes is the "batch" nature in which they are used, e.g. first slag is removed from the top of the blister copper, then blister copper itself is removed in a separate operation. Continuing with the example of slag and blister copper, subsequently the volume of blister copper from the furnace is allowed to increase such that the slag level is raised to a level for another removal operation, and the cycle repeats. This constant fluctuation in the level of slag and blister copper in the settling zone of the furnace increases the wear on the refractory that is exposed to this repeated change in elevation. Moreover, the metallurgy of the operation can also be affected by variation in the melt volumes.